Abstract
Starting in the 1950s and continuing into the early 1970s, a number of foil irradiations and fission chamber measurements were made in a variety of Fast critical assemblies at Los Alamos National Laboratory. These include (i) Godiva, a bare HEU spherical assembly; (ii) Flattop-25, a spherical assembly consisting of an HEU core and a natural uranium reflector; (iii) Jezebel, a bare 239 Pu assembly; and (iv) Flattop-Pu, a spherical assembly consisting of a 239 Pu core and a natural uranium reflector. In most instances the irradiations occur at or near the center of the assembly, but in selected instances data were obtained for a radial traverse extending into the Flattop reflector region. Measurements were made for a number of threshold reactions, including 45 Sc(n,2n)44m Sc, 51 V(n ,α)48 Sc, 75 As(n,2n)74 As, 89 Y(n,2n)88 Y, 90 Zr(n,2n)89 Zr, 103 Rh(n,2n)102g Rh, 107 Ag(n,2n)106m Ag, 169 Tm(n,2n)168 Tm, 175 Lu(n,2n)174 Lu, 191 Ir(n,2n)190 Ir, 197 Au(n,2n)196 Au, 203 Tl(n,2n)202 Tl, 204 Pb(n,2n)203 Pb and 238 U(n,2n)237 U. Fission ratio data for 238 U(n,f)/235 U(n,f) and 239 Pu(n,f)/235 U(n,f) were also obtained. We report C/E values from MCNP6 calculations using ENDF/B-VII.1 and IRDFF-v1.03 cross section data.
Highlights
We compare predicted versus measured reaction rate spectral indices for a variety of threshold reactions in the Los Alamos Godiva, Jezebel, Flattop-25 & -Pu and Big-10 critical assemblies
Godiva is a bare sphere of highly-enriched 235U (HEU), Jezebel is a bare sphere of ∼95 weight per cent 239Pu, the Flattop assemblies contain either an HEU or a 239Pu core surrounded by a thick natU reflector and Big-10 is a large heterogeneous uranium cylinder whose average enrichment is 10%
We note the International Criticality Safety Benchmark Evaluation Project (ICSBEP) benchmark evaluation used in our calculations
Summary
We compare predicted versus measured reaction rate spectral indices for a variety of threshold reactions in the Los Alamos Godiva, Jezebel, Flattop-25 & -Pu and Big-10 critical assemblies. Note that the Jezebel benchmark, Pu-MET-FAST-001 in ICSBEP nomenclature, was recently re-evaluated. The comparisons to experiment focus on ENDF/B-VII.1 cross sections [2], but the recently released IAEA’s IRDFF-v1.03 [3] are used in a few cases.
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